Set-Mailbox
Failed
Error:
Couldn't find object "domain.local/retired/staff/personsname". Please make sure that it was spelled correctly or specify a different object. Reason: The recipient domain.local/retired/staff/personsname isn't the expected type.

The solution was to open ADSI Edit, navigate to the user I was trying to grant the permissions for, find the publicDelegates setting, and remove the user being referenced. Problem solved. :)